1. Field of the Invention
The present disclosure generally relates to analog signal processing circuits and in particular but not exclusively to an attenuation cell having an attenuation factor control device for applying dB-linear control according to a control value, voltage or current.
2. Description of the Related Art
Attenuation cells are frequently used in analog signal processing circuits. When associated with an amplifier, an attenuation cell makes it possible to obtain variable gain, as illustrated in FIG. 1 where a cell 1 is combined with an amplifier 2—of the transconductance type—which receives a differential input signal Vin. By varying the current circulating in the load, made up of two resistors 3 and 4, the gain of the whole circuit can be controlled.
In many applications, it is desirable to have gain that is linear with control, expressed directly in decibels from a desired input, current or voltage. In the most recent wireless telecommunication techniques, based on the Wideband Code Division Multiple Access (WCDMA) principle or on the Code Division Multiple Access (CDMA), the gain of internal circuits of a mobile telephone communicating with a base station is controlled with control data from the base station. The performances of the telephone depend directly on the precision with which the gain set by the base station can be controlled.
In wireless telephony applications, as well as in many other applications, it is desirable to be able to precisely control amplifier gain, such control being in addition independent of temperature and independent of the dispersion of the characteristics resulting from the manufacturing method. Under these conditions, calibration tasks and telephone development are greatly simplified.
Generally, in known circuits, the linear relation between gain, in decibels, and desired input is established by means of a second order approximation, which leads to very limited precision. With this technique, accuracy hardly ever exceeds 1%.
U.S. Pat. No. 6,084,471 entitled “SOFT-LIMITING CONTROL CIRCUIT FOR VARIABLE GAIN AMPLIFIER” by Robert N Ruth and al. describes an example of an amplifier gain control circuit based on a mechanism that is both analog and digital, using a digital signal processor.
U.S. Pat. No. 6,124,761 entitled “VARIABLE GAIN AMPLIFIER WITH GAIN LINEAR WITH CONTROL VOLTAGE” by Trevor Robinson et al. describes another known example of an attenuation cell based on the use of a diode through which flows a current in order to carry out linear gain control.
It is observed that known attenuation control cells are limited with regard to the precision of the attenuation factor, which also largely depends on operational temperature. It would be desirable to have an attenuation control cell allowing a high degree of accuracy on a broad temperature range, larger than 100 degrees Celsius, for example.